1. Field of the Invention
The invention relates generally to a non-return braking arrangement for a device for the operation of a mobile device, and in a particular embodiment to control the flight of an aircraft.
The invention finds a particularly advantageous application in the context of the actuation of an element for adjusting the horizontal plane on an aircraft (the elevator aileron), though other applications are also possible.
2. Discussion of Related Art
Such a non-return device is more commonly known as a “no-back” device.
The actuation of a mobile element on an aircraft can be effected by means of a worm-screw actuating system generally with two attachments, a first attachment known as the aircraft attachment by which the actuating screw is associated with the structure of the aircraft, and a second attachment known as the structure attachment by which the actuating screw is attached by a nut to the mobile element to be actuated.
Generally, the role of a no-back device is to eliminate the effect of any force which, acting on the nut, tends to oppose or to favor the normal rotation of the actuating screw.
If a load were allowed to act against the input torque applied to the actuating screw, it would then be necessary to apply an even larger input torque requiring, in particular, a larger and heavier input motor.
Similarly, an assisting torque acting on the input torque applied to the actuating screw would contribute energy but would also cause difficulties regarding speed regulation as well as in maintaining the position of the actuating device.
A no-back device is used in particular to prevent the actuating screw from starting to rotate under the effect of an axial aerodynamic load of the traction or compression type, acting on the nut. Any force applied to the actuating screw, whose effect is to assist the input torque or to oppose this torque, is then eliminated.
A more detailed description of the operation of a conventional no-back device is provided below with particular reference to FIGS. 1 to 3.
A failure of a no-back device is potentially dangerous, since the shaft of the actuating screw could then be driven by the aerodynamic forces acting on the mobile element to be actuated. The mobile element would then not be held in the desired position and could flutter, rendering the aircraft unstable.
Also proposed was an actuating device in which the function of the no-back device is redundant and can be over-ridden, in the event of failure of the latter, by a secondary sprocket chain which holds the position of the nut by means of a control loop.
Such a device, though affording security, nevertheless gives rise to the risk that an operation that depends only on the secondary sprocket chain, after failure of the primary no-back function, would prevent the failure from being detected.
The actuating device is then no longer providing its additional level of security, and is robbed of its original purpose.
In addition, a simple break in the secondary sprocket chain would result in the loss of the attachment between the structure of the aircraft and the mobile element to be actuated, thus leading to a catastrophic situation.
There is therefore a requirement that a failure (of the primary function) of a no-back device be detected, in particular in order to avoid any operation, that depends on the secondary chain only, according to a dormant failure mode.